Those skilled in the arts of antennas, antenna arrays and beamformers know that antennas are transducing devices which transduce electromagnetic energy between unguided- and guided-wave forms. More particularly, the unguided form of electromagnetic energy is that propagating in “free space,” while guided electromagnetic energy follows a path established by a “transmission line”. Transmission lines include coaxial cables, rectangular and circular hollow waveguides, dielectric microstrip or stripline paths, and the like. Historically, the guided-wave port of an antenna has been termed a “feed” port, regardless of whether the antenna operates for signal transmission or reception. The beam characteristics of an antenna are established, in part, by the sizes of the radiating portions of the antenna relative to the wavelength of the electromagnetic signal. In general, small antennas produce broad or nondirective beams, and large antennas produce small, narrow or directive beams. When more directivity (larger gain and/or narrower beamwidth) is desired than can be achieved from a single antenna, several antennas may be grouped together into an “array” and fed together in a phase-controlled manner, in order to generate the beam characteristics of an antenna which is larger than that of any single antenna element. The devices which control the phases and apportionment of signals to (or from) the antenna transducing elements are termed “beamformers.” In general, a beamformer includes at least one feed port and a plurality of element ports. In a transmit mode, the signal is applied to the feed port and is distributed by the beamformer to the antenna transducing elements for transmission as an unguided electromagnetic signal in free space. In a receive mode, the unguided electromagnetic signals in free space are received by the antenna transducing elements and are combined in the beamformer to produce a signal at the feed port. A salient advantage of sophisticated beamformers is that they may include a plurality of feed ports, each of which distributes the electromagnetic signal in such a fashion that a plurality of different antenna beams may be generated simultaneously.
Array antennas are well known for various communication and sensing applications, and exhibit advantages over shaped-reflector antennas in that scanning (changing the pointing direction) of the beam or beams may be performed essentially instantaneously by electronic means, without inertia problems associated with moving a shaped reflector which has mass. In order to perform its role of setting the direction of the beam of an antenna array, a beamformer must set the element-to-element phase shifts of the individual signals being transduced by the array of transducing elements. So long as the beam direction or shape does not have to be changed during the operation of the antenna, fixed phase shifting devices may be used in the beamformer. Fixed phase shifting devices may be passive electronic devices which are ordinarily very reliable. When the direction or shape of the antenna beam must be changed during the operation of the antenna, controllable phase shifting devices may be used in the beamformer. Controllable phase shifting devices may be analog or digitally controlled devices. Currently available phase shifting device designs prefer multibit digitally controlled types because they can be controlled by simple digital signals, and because the amounts of phase shift can be quickly and accurately set. Controllable phase shifting devices may be active electronic devices, whether analog or digital, which are more subject to failure than are electronically passive phase shifting devices.
“Active array” antennas may have both a power amplifier and a low-noise amplifier associated with each transducing element. In a transmission mode, the beamformer distributes low-amplitude signals which are amplified by the associated power amplifiers and then are transmitted into free space by the transducing elements. The distribution of signals at low-amplitudes in the beamformer during transmission mode is desirable because it avoids ohmic heating losses which would otherwise occur in the beamformer if high power signals were distributed. In a reception mode, the signals received by each transducing element are amplified by the associated low-noise amplifiers prior to being combined in the beamformer. The amplification of the signals prior to combining them in the beamformer during reception mode is desirable because it establishes the receiver noise figure prior to incurring ohmic losses in the beamformer.
Control of the beam of an array antenna may involve “weighting” of the relative amplitudes of the signals which are applied to (or received by) the antenna transducing elements. The purposes for weighting are to establish the directivity and shape of the main beam of the antenna spatial pattern and also to establish the levels of the sidelobes (unwanted subsidiary lobes). A uniform weighting provides each antenna transducing element of the array with equal weight, meaning that in a transmission mode of operation all antenna transducing elements are provided with equal amplitude signals, and in a receive mode of operation equal amplitude signals from the individual antenna transducing elements are combined in the beamformer. Uniform weighting may be desirable when maximum directivity or antenna gain is desired. However, uniform weighting tends to produce high sidelobe levels, which are about −13 dB to −18 dB (deciBels) relative to the peak directivity of the main beam. Sidelobes (unwanted subsidiary lobes) are an inherent characteristic of directive array antennas which consist of multiple transducing elements, and may cause the transmission of power in directions other than the desired direction, or may also cause the reception of signals from directions other than the desired direction. Such transmissions in undesired directions may cause mutual interference to other antennas in close proximity, may enable the detection of the source of the signals by hostile forces, and may also cause a waste of transmitter power. The reception of signals from undesired directions may expose the antenna to mutual interference from other signal sources within close proximity and to jamming by hostile forces. The prior art controls the sidelobe levels of an array antenna by weighting or adjusting the array weighting function. Examples of prior art weighting functions which produce low sidelobe levels in the absence of transducing element failures include raised-cosine weighting, Dolph-Tschebyscheff weighting, and Taylor weighting functions. Weighting in an array antenna is ordinarily a function of the beamformer. Weighting of the levels of signals in the beamformer may be accomplished by signal attenuators and/or by power dividers (or power combiners in receive mode) which are selected to give the desired weighting. For some applications, variable gain or attenuation may be used and controlled in order to rapidly change the beam pointing direction, shape or other antenna pattern characteristics such as sidelobe levels.
Each transducing element of an array antenna may be associated with a “power” amplifier for use in a transmit mode, a “low-noise” amplifier for use in a receive mode, and a phase shifting device or devices for use in both transmit and receive modes. The power amplifier, the low-noise amplifier, and the phase shifting device or devices which are associated with each antenna element are often combined into a “transmit-receive” (TR) module. This module, in addition to containing the amplifiers and phase shifting device or devices, may also contain controllable gain devices, radio-frequency (RF) switches for switching between transmit and receive modes of operation, controls for the switches, and power supplies for the various controls and active electronic devices. The power amplifiers, low-noise amplifiers, digitally controlled phase shifting devices, and other active electronic devices which are contained in the TR modules, notwithstanding efforts to improve-reliability, may be subject to various forms of failure.
In the past, the term “radio frequency” was interpreted to mean a limited range of electromagnetic frequencies, such as, for example, the range extending from about 20 KHz to 2 MHz. Currently, the term “radio frequencies” as now understood extends over the higher electromagnetic frequency spectrum, including those frequencies in the “microwave” and “millimeter-wave” regions. Many of these electromagnetic frequencies are very important for military and commercial purposes, as they include the frequencies at which radar systems, global positioning systems, satellite cellular communications and ordinary terrestrial cellphone systems operate.
The failure of a TR module generally stops the operation of the associated antenna transducing element, and may be viewed as being a failure of the antenna transducing element itself. Failures in the TR modules of an array antenna may occur, in general, in the transmit signal path, in the receive signal path, and/or in the phase shifting device signal or control paths. Thus, in a transmit mode of operation, different transducing elements of the antenna array may fail than fail in a receive mode of operation. Prior art weighting functions are designed for array antennas in which all antenna transducing elements are operational. Failures of one or more transducing elements of an array may significantly degrade the antenna sidelobe levels which are obtained with prior art weighting functions.
Thus, active array antennas, such as are currently being used and which will be used in future radar, communication and commercial applications, provide significant performance benefits relative to passive array antennas. The placement of a TR module close to each transducing element tends to reduce signal losses (beamformer losses) and improve sensitivity for detecting small signals during receive modes, and tends to increase transmitted signal power levels during transmission modes. However, these TR modules contain active-electronic devices which may cause the antenna RF (Radio Frequency) circuits to have significantly higher failure rates as compared to RF circuits which have only passive electronic devices. And while these failures are not expected to significantly degrade certain antenna performance parameters, such as directivity, other performance parameters may be significantly degraded, such as antenna pattern sidelobe levels. Both military and commercial active-array antennas are at risk of not meeting expected requirements for low antenna pattern sidelobe levels due to electronic element failures.
U.S. Pat. No. 4,359,740, issued Nov. 16, 1982 in the name of Frazita, recognizes the possibility of failure of a phase shifting device of an array antenna. The failure is detected, and the signal energy to the failed phase shifting device is cut off, at least at certain beam angles. An FFT-based aperture monitor is described in U.S. Pat. No. 4,926,186, issued May 15, 1990 in the name of Kelly et al., which reveals faulty elements or phase shifting devices. U.S. Pat. No. 5,512,900, issued Apr. 30, 1996 in the name of Parkin et al., describes an instrument landing system in which antenna sidelobe levels are monitored, and which shuts down the system when the sidelobe level is deemed to be unacceptably degraded. U.S. Pat. No. 6,140,976, issued Oct. 31, 2000 in the name of Locke et al. describes an array antenna which has more elements than required in order to meet normal requirements, and which replaces a failed element to maintain desired performance levels. U.S. Pat. No. 4,623,381, issued Oct. 9, 1990 in the name of Helbig, describes failed-element processing in the context of systolic processing. Other schemes involve increasing the size of the antenna array over that which would otherwise be necessary in order to reduce the relative significance of individual transducing element failures, and reconstructing signals from failed transducing elements by applying known signal angle-of-arrival information and interpolation to signals from nearby non-failed transducing elements.
Improved antenna arrays and processing therefor are desired.